Possible excesses of leukemia and brain cancer have been identified in case-control studies of electrical workers presumably exposed to time-varying (60 Hz) magnetic fields. These case control studies have relied upon surrogate measures, such as job description, for exposure estimation. The proposed study intends to utilize exposure data currently being collected as a part of a study of brain cancer in a large metal machining and assembly plant. Magnetic field exposure assessment in this environment is problematic due to the large degree of exposure variability associated with worker movement and the nature of the fields which decrease exponentially with distance from the source. Exposure data will be collected using a time integrating dosimeter and a survey meter. The dosimeter will be used to collect real-time exposure data during the workshift. The survey meter will be used to collected detailed area or spot measurements to evaluate spatial variability. Averages of the spot measurements will be compared to the time-integrated estimates to determine if the use of the dosimeter is necessary. In addition the output from the dosimeter will be used to calculated assorted exposure indices such as peaks and threshold exceedences. It is stated that, the data collected for this study, along with recently developed methods for characterization of air contaminant variability, will allow the investigators to recommend exposure assessment strategies for future studies of magnetic field exposure.